1. Field of the Invention
The present invention relates generally to a cellular wireless communication system, and more particularly to a method and an apparatus for transmitting/receiving system information through a Broadcast Channel (BCH).
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) technology is widely applied in the recent development in broadcasting and mobile communication technology. OFDM technology eliminates interference between multi-path signal components existing in a wireless communication channel, guarantees the orthogonality between multiple access users, and enables efficient use of frequency resources. Therefore, OFDM technology is more useful for high speed data transmission and wideband systems than Direct Sequence Code Division Multiple Access (DS-CDMA) technology, such as Wideband Code Division Multiple Access (WCDMA) or CDMA 2000.
FIG. 1 illustrates a structure of an OFDM signal in a frequency and time domain.
Referring to FIG. 1, one OFDM symbol 100 occupies N number of sub-carriers 102 in view of the frequency domain. The sub-carriers 102 are simultaneously transmitted in parallel after being loaded on modulation symbols (or sub-carrier symbols) of transmitted information, respectively. As described above, the OFDM technology is multi-carrier transmission technology, which can transmit data and control information by multiple sub-carriers. In an OFDM-based communication system, each physical channel includes at least one sub-carrier symbol 104.
One important characteristic of an OFDM-based cellular wireless communication system for providing a high speed wireless data service is the ability to support a scalable bandwidth. A system based on the scalable bandwidth can have various system bandwidths, such as 20/15/10/5/2.5/1.25 MHz. Service providers can provide a service after selecting one of the bandwidths for each cell, and types of User Equipment (UE) may range from those capable of providing a service with a reception bandwidth of maximum 20 MHz to those capable of supporting only a reception bandwidth of at most 1.25 MHz.
In a scalable bandwidth-based system, a UE accessing the system for the first time is required to make a successful cell search even without information about the system bandwidth. Through the cell search, the UE acquires a cell ID and synchronization between a transmitter and a receiver for demodulation of data and control information. The system bandwidth can be obtained from either a Synchronization Channel (SCH) during the cell search or a Broadcasting Channel (BCH) after the cell search, which is a common control channel for transmission of system information. The BCH is a channel for transmitting system information of a cell which the UE accesses, and is demodulated after the UE finishes the cell search. The UE successfully acquires initial synchronization of a particular cell by searching the cell through the SCH and then acquires the system information for the cell by receiving the BCH. That is, by reading the BCH, the UE acquires system information necessary for receiving a data channel and other control channels for each cell, such as a cell ID, a system bandwidth and channel setup information.
FIG. 2 illustrates examples of frequency resource mapping of the SCH and the BCH according to a system bandwidth in a conventional system supporting a scalable bandwidth.
In FIG. 2, the vertical axis corresponds to a frequency domain, which illustrates frequency mapping according to system bandwidths of 1.25/2.5/5/10/15/20 MHz. As shown, the sequence of the SCH 204 and the system information of the BCH 206 are transmitted in the middle of the system band with a bandwidth of 1.25 MHz regardless of the system bandwidth. Therefore, the UE searches a Radio Frequency (RF) carrier 202, which is a central frequency of the system band, regardless of the system bandwidth, and performs cell search for the central band of 1.25 MHz centering the RF carrier 202, thereby detecting the SCH 204 and acquiring initial synchronization for the system. Further, after the cell search, the UE demodulates and decodes the BCH 206 transmitted in the same 1.25 MHz band, thereby obtaining the system information.
FIG. 3 illustrates a case in which each of the SCH and the BCH has a changing transmission bandwidth according to the system bandwidth. That is, the sequence of the SCH 304 and the system information of the BCH 306 are transmitted with a bandwidth of 1.25 MHz when the system bandwidth does not exceed 2.5 MHz, while the sequence of the SCH 304 and the system information of the BCH 306 are transmitted with a bandwidth of 5 MHz when the system bandwidth has a value of at least 5 MHz. This is in order to transmit the SCH sequence and the BCH system information by using a wider band in the case of a larger system bandwidth, thereby improving performance in cell search and system information reception.
In a system supporting the scalable bandwidth, it is necessary to design the channels such that a UE having a reception bandwidth smaller than the system bandwidth can successfully perform SCH search and BCH reception from neighboring cells even when the UE receives a service in a part of the system bandwidth.